Visibility in an ocean environment has always been a problem. The range of active underwater optical viewing is limited to a few yards. In turbid water the useful range is even shorter.
The invention described herein relates to an acoustic hologram reconstructor, a major component of a holographic acoustic imaging system, which accomplishes reconstruction of complex acoustic holograms to obtain an image of an underwater scene. In brief, the data input from this reconstructor is a two-dimensional array of complex numbers which may be represented, digitally or in analog form, as a set of voltages stored on capacitors, which form the complex acoustic hologram. The device performs a two-dimensional Fresnel transform (focused Fourier transform) yielding a video type signal, an intensity pattern, which contains the image information. A Fresnel transform is similar to a conventional Fourier transform, but it includes a focusing correction term, to compensate for the distortion due to the acoustic signal source being too close to the hydrophone array from which the acoustical hologram is developed.
Acoustic imaging is a means of viewing objects underwater which is useful when optical viewing is limited by either water turbidity or attenuation. It encompasses several techniques of transmitting, receiving, and processing acoustic signals to create images which resemble the objects. This description hereinbelow concentrates on active acoustic systems, but the techniques may be extended to passive systems.
The basic goal of imaging is to display visually the intensity distribution reflected from a given scene. For those acoustic systems using sensitive hydrophone transducers, this requires four basic functions: acoustic-to-electronic transduction, spatial processing, detection, and display. The three basic types of acoustic imaging systems -- focused, beamformed, and holographic-- differ in the order in which the first three functions are performed.
For focused acoustic imaging, the functions are performed in this order: spatial processing, transduction, detection, and display. The spatial processing is accomplished with an acoustic lens and the transduction is done with a hydrophone array. The signal at each element of the array comes from a different angle in the field of view and is sensed with a square law detector to determine the intensity at that angle. Focused acoustic imaging systems differ in the ways in which the complexity is reduced by the use of smaller arrays with fewer detectors and by means of scanning the image or the array.
Beamformed systems perform the transduction of the acoustic signal first, with spatial processing and detection following in order. The spatial processing is achieved with various delaying and summing networks. The way in which the time delays and sums are implemented depends upon the acoustic frequency and the technological state-of-the-art. Three basic types of beamformed systems are evident: multiple beam, mechanically scanned beam, and electrically scanned beam. The differences between the systems occur from trade-offs between hardware complexity and time. Multiple beam systems are complex, but they are able to form a full image at one time, allowing flexible signal-to-noise gain through integration in the detectors. Scanned beamformers achieve simplicity by looking at only one beam at any given time. In order to achieve signal-to-noise gain through integration over time, the scanner must stay on a given beam longer, increasing the time required to obtain an image. During this time energy coming from other beams is being ignored, resulting in a decrease in efficiency.
For holographic acoustic imaging (FIG. 1) transduction is followed by detection and then spatial processing. In order to accomplish the spatial processing, the detectors must obtain amplitude and phase information from the signal at each hydrophone. This is accomplished by a channel processor with an electronic reference wave input.
At each hydrophone of the hydrophone array, a complex signal is sensed, that is a signal having an amplitude and a phase angle. The channel processor changes the acoustic signal into an electrical signal which is also complex.
Spatial processing is achieved by a reconstructor which can be implemented by various means. Synthetic aperture holographic systems and filled-array systems differ in the trade-off between complexity and time. With synthetic aperture systems, the amplitude and phase from only a portion of the acoustic field are measured at any given time. The acoustic field is scanned, either electrically or mechanically, and the holographic data is stored from scan to scan for reconstruction, when data from the entire field is obtained.
Summarizing the comparison, each of the three methods of acoustic imaging is likely to perform best for a specific application. Focused imaging systems are preferable at acoustic frequencies above 1 MHz where the short wavelength allows small lenses and short focal lengths. At these high frequencies, the scale lengths are so small that there is no room for the electronics presently required for holographic systems. With further development, forms of electrically scanned beam-formed systems may become competitive. At lower frequencies the scale lengths are so large that focused systems are cumbersome. Then holographic systems and scanned beamformed systems become desirable. Multiple beamformed acoustic imaging systems would be cumbersome to build because of the many interconnections required for spatial processing. (N.sup.2 connections from each element of an N .times. N array are required to form N.sup.2 beams.) For holographic systems, N.sup.2 parallel channel processors measure holographic data. The complex spatial processing is done in the reconstruction process. Holographic systems also offer greater flexibility in post-detection processing, or image enhancement. This is particularly relevant to acoustic imaging, when low-resolution images of mirror-like targets are difficult to recognize.